1. Field of the Invention
The present invention relates to a method of processing image data and an apparatus for recording image data advantageously applicable to a digital still camera or the like for forming image data and recording the image data.
2. Description of the Background Art
Conventional solid-state image pickup devices, e.g., CCD (Charge Coupled Device) image sensors had a dynamic range far narrower than a latitude particular to silver halide photo-sensitive type of films and other traditional photosensitive materials. Today, a solid-state image pickup device with a broad dynamic range is available because of the rapid progress of semiconductor fabrication technology and that of broad dynamic range shooting technology, e.g., multiple exposure shooting.
Various schemes have been proposed for promoting the effective use of raw data having a broad dynamic range. Japanese patent laid-open publication No. 90380/1994, for example, discloses a procedure that compresses image data with a knee curve by analog processing and then expands the compressed image data with an inverse knee curve by digital processing to thereby obtain image data having a broad dynamic range.
To record image data representative of a scene picked up and having a broad dynamic range in the form of a digital picture signal, use is made of a defacto, standard file format, e.g., BMP (Bit MaP)), TIFF (Tag Image File Format) or JPEG (Joint Photographic Experts Group) format. Today's defacto, standard file format has a dynamic range whose unit data (pixel data in the case of a picture) consists of eight bits because an eight-bit dynamic range is a defacto, standard dynamic range adopted by the above typical formats. In the future, the standard file format may be replaced with another format having a different standard dynamic range.
At the present stage of development, for conversion to the standard file format, pixel data represented by an RGB (Red, Green and Blue) model or similar data model must be rendered by a preselected number of quantizing levels, i.e., eight-bit levels at the present stage. As for an RGB model, for example, an R, a G and a B channel constituting pixel data each are represented by eight-bit levels, e.g., 255 quantizing levels.
However, the problem with the conversion using a preselected number of quantizing levels is that the original image data with a broad dynamic range looses essential part thereof having high accuracy, i.e., represented with a great number of quantizing levels. The high-accuracy data lost cannot be recovered in the event of reproduction to be effected by software processing later. Particularly, software processing for enhancing the gain of the dark portion of a picture cannot be executed with accuracy above eight bits.
Assume that data with a great number of quantizing levels, e.g., twelve-bit data is recorded in order to effectively use the entire data having a broad dynamic range. Then, such data do not adapt to the standard file format described previously and cannot be transformed to the JPEG or similar compressed format. Moreover, the data with the great number of quantizing levels occupy a broad space in a memory.